The only way to lighten the stain color is to remove the existing stain and start over with a lighter color. Fortunately this isn't as hard as you think. You have a couple of choices depending if you applied a clear finish over the stain.

If the door is stained and not finished, use lacquer thinner to dissolve the stain. Work in sections and use a stiff nylon brush to work the lacquer thinner into the embossed texture.

The lacquer thinner will dissolve the stain quickly and also evaporate very fast. You will have to flood an area with the thinner then quickly wipe it with a white rag. Repeat until all of the stain is removed.

If you applied a clear finish to the fiberglass door I recommend using a paint stripper instead of the lacquer thinner. Choose a paint stripper that is appropriate for the type of clear finish you used.

Both of these options should be preformed with the door on sawhorses and with good ventilation.